The present invention relates to transmission line testing, and more particularly to a method of determining the nature of a transmission discontinuity (fault) and its severity.
In transmission line testing it is very important to know not only the distance to a transmission discontinuity, but also the nature of the fault causing the discontinuity and its severity. For frequency domain based measurements, due to the frequency selective nature of most reflection responses, the distance-to-fault (DTF) result sometimes does not reflect the actual severity of the problem. For example a transmission line with a severe corrosion problem when stimulated with a step-function waveform shows a large reflection, as shown in FIG. 1b, but may only show a small reflection when stimulated with an impulse response or with swept tones of no low frequency components, as shown in FIG. 1a, which is hardly distinguishable from some non-harmful reflections. Where a test instrument uses a simple impulse-like signal source by stepping a sinusoid through a frequency range of interest, such as from 24 MHz up, it has the potential of missing the severity of built-up deterioration.
In an antenna system the most common problems are corrosion and watering. FIGS. 2a and 2b show the types of terminating impedances associated with each of these problems, with FIGS. 3a and 3b showing the frequency responses for the resultant reflections. Different terminating impedances have different frequency responses. When there are more than one reflection surface in the transmission line, the characteristic of the frequency response profile for the measured reflection is usually hard to categorize. FIGS. 4a and 4b show the frequency response and the reflection magnitude response of a system with both Type-I and Type-II impedances. As seen the spectral profile in FIG. 4a is neither the Type-I nor the Type-II profile shown in FIG. 3a or 3b. 
What is desired is a method of determining not only the distance to a transmission discontinuity but also the nature of the fault causing the discontinuity and its severity, especially in situations where the characteristic of a frequency response profile for the measured reflection is difficult to categorize.